Error Handling (GNU Fortran Compiler Internals)

### 2.2 Error Handling ¶

The GNU Fortran compiler’s parser operates by testing each piece of source code against a variety of matchers. In some cases, if these matchers do not match the source code, they will store an error message in a buffer. If the parser later finds a matcher that does correctly match the source code, then the buffered error is discarded. However, if the parser cannot find a match, then the buffered error message is reported to the user. This enables the compiler to provide more meaningful error messages even in the many cases where (erroneous) Fortran syntax is ambiguous due to things like the absence of reserved keywords.

As an example of how this works, consider the following line:

    IF = 3

Hypothetically, this may get passed to the matcher for an `IF` statement. Since this could plausibly be an erroneous `IF` statement, the matcher will buffer an error message reporting the absence of an expected ‘`(`’ following an `IF`. Since no matchers reported an error-free match, however, the parser will also try matching this against a variable assignment. When `IF` is a valid variable, this will be parsed as an assignment statement, and the error discarded. However, when `IF` is not a valid variable, this buffered error message will be reported to the user.

The error handling code is implemented in `error.cc`. Errors are normally entered into the buffer with the `gfc_error` function. Warnings go through a similar buffering process, and are entered into the buffer with `gfc_warning`. There is also a special-purpose function, `gfc_notify_std`, for things which have an error/warning status that depends on the currently-selected language standard.

The `gfc_error_check` function checks the buffer for errors, reports the error message to the user if one exists, clears the buffer, and returns a flag to the user indicating whether or not an error existed. To check the state of the buffer without changing its state or reporting the errors, the `gfc_error_flag_test` function can be used. The `gfc_clear_error` function will clear out any errors in the buffer, without reporting them. The `gfc_warning_check` and `gfc_clear_warning` functions provide equivalent functionality for the warning buffer.

Only one error and one warning can be in the buffers at a time, and buffering another will overwrite the existing one. In cases where one may wish to work on a smaller piece of source code without disturbing an existing error state, the `gfc_push_error`, `gfc_pop_error`, and `gfc_free_error` mechanism exists to implement a stack for the error buffer.

For cases where an error or warning should be reported immediately rather than buffered, the `gfc_error_now` and `gfc_warning_now` functions can be used. Normally, the compiler will continue attempting to parse the program after an error has occurred, but if this is not appropriate, the `gfc_fatal_error` function should be used instead. For errors that are always the result of a bug somewhere in the compiler, the `gfc_internal_error` function should be used.

The syntax for the strings used to produce the error/warning message in the various error and warning functions is similar to the `printf` syntax, with ‘`%`’-escapes to insert variable values. The details, and the allowable codes, are documented in the `error_print` function in `error.cc`.